The present invention relates to means for effecting temperature-compensation of focal length in an optical assembly.
Certain optical assemblies such as those found in laser output scanners require a stable monochromatic collimated light beam provided by a laser diode and a collimating lens. For adequate optical performance, the beam source must maintain a predetermined beam quality over a wide ambient temperature range. In conventional apparatus, the beam source and lens are mounted in a mechanical structure that attempts to maintain the beam focal length while the apparatus undergoes temperature-induced structural changes. Hence, the thermal compensation is effected either passively or actively.
Passive compensation systems rely on the differences in coefficients of thermal expansions of the various elements in the optical system such that there is minimal net focus shift with temperature. The conventional approach is to employ concentric tube systems, which, if constructed from common materials, are too large or bulky. For example, U.S. Pat. No. 4,730,335 discloses a series of interlocking tubes each carrying a single optical element of an optically-pumped solid-state laser. Such an apparatus is too large to be suitable for many applications.
U.S. Pat. No. 4,861,137 discloses a mounting device for a lens used to refract a light beam, wherein the lens is supported by doughnut-shaped bimetal components which deflect according to the ambient temperature. This approach can be moderately effective for supporting lenses having diameters under approximately 1 cm. However, the motion of the bimetal components is not fully constrained, and the lens undergoes a tilted and/or tipped motion relative to the optical axis. The boundary conditions inherent in the disclosed design thus allow linear motion of the lens over but a very small range of motion. The disclosed approach is therefore unsuitable for effecting fine adjustment of lens position over a large range of motion. Moreover, the disclosed bimetal components suffer from a temperature gradient over their radial dimension; thus rendering the temperature compensation less accurate and less effective than desireable.
Prior art approaches that rely on active control (for example, wherein heating elements or thermoelectric coolers are used) have several inherent disadvantages. For example, a thermoelectric cooler is employed in the apparatus disclosed in U.S. Pat. No. 4,604,753 to stabilize the output power and wavelength of a laser diode beam source; U.S. Pat. Nos. 4,656,635 and 4,993,801 disclose a beam source wherein a thermoelectric cooler is employed to control the operating temperature of the entire head. These apparatus are expensive to construct and require an external power source.